Teeth outside the jaws in ratfish: an alternate view

Cohen, Coates and Fraser 2025
published a study on the tenaculum = clasper on male chimaeras = ghost sharks = ratfish = rabbitfish. The publicity (see below) focused on ‘forehead teeth to help them have sex.’

The team looked at several extant ratfish
and one, Helodus simplex, from 315 mya, which the authors were able to µCT scan.

DGS colors are applied here and here.
Click on the links to see the taxa and data.

Helodus simplex
(Agassiz 1838, Coates et al 2021; Cohen, Coates and Fraser 2025, Late Devonian to Early Permian, 315 mya; 30cm) nests basal to chimaeroids, derived from Isurus, the extant mako shark in the large reptile tree (LRT, 2340 taxa).

Unfortunately, the authors did not extend their study to include Isurus.

Cohen, Coates and Fraser considered the tooth whorl
at the tip of the slender nasal process homologous to oral teeth rather than modified skin denticles. They worked from a µCT scan for the fossil and studied juvenile and adult ratfish.

Here that hypothesis is reconsidered based on inclusion of pertinent taxa omitted from the 2025 paper.

In hypothesis number two
Helodus originally developed a spiral dermal ‘tooth whorl’, not as gender signal in a dimorphic species, but as protection for the otherwise fragile nasal tip in contact with the substrate where ratfish sensed buried prey.

Note: the nares were oriented anteriorly, directly in the wash of any substrate stirred up by the anteriorly-extended and down-turned slender, toothed nasal process, as practiced in the extant elephantfish, Callorhinchus, a basal ratfish in the LRT.

Looking back in time down the evolutionary ladder/bush,
this slender downturned process in Helodus and Callorhinchus was inherited from the much more robust, straight snout internal support of the mako shark, Isurus.

Unfortunately, the authors did not extend their study to include Isurus.

The authors considered the tooth whorl
on the claspers (= tenaculum = slender nasal process) to originally be part of the oral arcade. That would have made the teeth at its central tip homologs for premaxillary teeth. Given that identity the authors had to label the central teeth in the tooth row as vomer tooth homologs.

No related taxa have or had vomer teeth.
Plus in the LRT the closest relatives of Helodus all extend the nasal beyond the jaws.

Callorhinchus milii
(Lacepede 1798, 120cm in length) is the extant elephantfish. Slender nasal processes anchor the fleshy portion of the sensitive snout for sensing buried prey. Teeth are reduced to two pair in the upper jaw and one pair in the lower, all flat and broad. The dorsal fin spine can be elevated or depressed.

Origin of the Chimaeriformes = ratfish in the LRT
Rather than a primitive shark/ratfish split, the LRT recovered a ratfish origin from Isurus the mako shark. Here the nasal cartilage evolves to become an olfactory sensitive prey detector in Late Devonian Helodus and extant Callorhincus.

Extant and bottom-dwelling Heterodontus is related, but loses the slender, ventrally extended nasal process, replacing it with literally nothing = a blank, open, depressed area.

In agreement with Cohen, Coates and Fraser,
the tenaculum stopped being used for sweeping the substrate, as it over time elevated to the forehead only in males. That original touch-sensitivity was repurposed to contact females. The tenaclum teeth became reduced as this dimorphic trait evolved.

Why DGS colors work better than labels
All pre-tetrapods = all fish in the LRT are colorized with tetrapod homolog colors. That avoids the otherwise unavoidable ‘track switching’ that would have happened because traditional fish diagrams often and otherwise use labels distinct from tetrapod labels – except for a few obvious homologs. This is due to bone splitting and fusing along cladistic lines. DGS colors can be seen to lump and separate various skull bone sets.

This interpretation took a year to figure out. Even so mistakes continue to be corrected given better data, as in Helodus.

References
Agassiz L 1838. Recherches Sur Les Poissons Fossiles. Tome III (livr. 11). Imprimérie de Petitpierre, Neuchatel 73-140.
Coates, MI et al (4 co-authors) 2021.The cranium of Helodus simplex (Agassiz, 1838) revised. In Janvier P, Denton JSS (Eds). Ancient Fishes and their Living Relatives. A Tribute to John G. Maisey (Pp 193-204).
Cohen KE, Coates MI and Fraser GJ 2025. Teeth outside the jaw: Evolution and development of the toothed head clasper in chimaeras. PNAS 122 (37) e2508054122 https://doi.org/10.1073/pnas.250805412

wiki/Helodus
wiki/Chimaera

reptileevolution.com/chimaera

Publicity
Ghost sharks grow forehead teeth to help them have sex

Source: https://pterosaurheresies.wordpress.com/2026/04/06/teeth-outside-the-jaws-in-ratfish-an-alternate-view/